Homemade bombs are the biggest killer of American troops, both in Iraq and Afghanistan. You hear them called IEDs, improvised explosive devices, and the Pentagon has spent about $14 billion to try to defeat them. This week, we're going to look at what the U.S. is doing to protect American troops from these bombs. In the first part of our series, NPR's Bruce Auster visited the Aberdeen Proving Ground, where troops and scientists are looking for ways to save American lives.

BRUCE AUSTER: Blowing things up is what they do here at Aberdeen. Live mine field, keep out. We're driving through this sprawling Army base, it's north of Baltimore, and we keep passing warning signs. They're posted everywhere. That sign says: Danger. Firing in progress. When red light is flashing, no admittance. So, of course, we're going in.

We're here at this test site, which is little more than an open field, and it's dotted with scarred blast walls.

Dr. SCOTT SCHOENFELD (Civilian Scientist Working with Army): And it looks like a disaster area. It's just�

AUSTER: That's Dr. Scott Schoenfeld. He's a civilian scientist here.

Dr. SCHOENFELD: It's a site where thousands of explosive devices have been set off over the years.

AUSTER: He's going to set off another one today. It's part of his research on what happens when bombs blow up and how they do their damage.

Dr. SCHOENFELD: That's been my life for the last five years. Constant new devices, understanding what they do, and it's a cycle.

AUSTER: That cycle works like this: In Afghanistan or Iraq, a roadside bomb goes off. The American side figures out how to outsmart that kind of IED, maybe by jamming the radio signal that triggered the bomb. Then, insurgents come up with a new design, and it's back to the drawing board.

Dr. SCHOENFELD: The key to understanding the IED, it's the I. It's the improvised part. There's no rules here. There's no set configuration. There's no one way that this was going to happen or be deployed against you.

AUSTER: Today, Dr. Schoenfeld is studying how one bomb design penetrates armor - not the kind you wear, but the kind on combat vehicles that American troops use in Afghanistan and Iraq.

AUSTER: The bomb is just outside. It's maybe 50 yards away, sitting on a stepstool on the other side of a blast wall.

Unidentified Man #1: Okay, here we go: Three, two, one�

(Soundbite of explosion)

AUSTER: After the area is secured and they're sure no more bomb fragments are raining down, we wander over with Dr. Schoenfeld to where the blast took place.

Dr. SCHOENFELD: Be careful. There's a lot of junk and debris in here.

AUSTER: Picture a thick black wall with a hole in the middle. The hole lets some debris fly through. On one side of the wall, where the IED had been on that stepstool, the stepstool has been obliterated.

This side of the chamber is pock-marked from all the times bombs have blown up. The other side of the wall is called the target chamber, and that's where the armor is that they're testing.

Dr. SCHOENFELD: So the fragments and debris are flying through here. We have the high-energy X-rays, and we can get a real-time image of how those are all interacting.

AUSTER: The X-rays can peer through the flame and smoke to capture the moment when the IED fragments hit the armor.

Dr. SCHOENFELD: And it's just like taking a picture at the dentist's office.

AUSTER: And that information about how bomb fragments hit armor lets the military to design better armor for its vehicles. But it's only one part of the IED fight.

Dr. SCHOENFELD: I mean, if all the other measures don't work�

Dr. SCHOENFELD: Don't work?

AUSTER: This is the last line�

Dr. SCHOENFELD: We are the last line of defense.

AUSTER: To understand how, what Dr. Schoenfeld does fits in with all the other work being done to stop IEDs. We visited a trade show in Virginia. That's where we met Erin Piateski. She works for the organization the Pentagon set up to try to stop IEDs, and she's here meeting contractors who are pitching their ideas for doing that.

Ms. ERIN PIATESKI (Works with Pentagon): What tends to happen in the IED world is they start out extremely simple, and they become more sophisticated. And then as our countermeasures defeat the very sophisticated sorts of IEDs, they tend to go back to something very simple.

AUSTER: It turns out simple is often harder to beat. Piateski has with her some small boxes filled with what look like throwaways you'd find in your basement. They're sample IEDs. One design uses a pair of old cell phones. One phone would be placed with the explosive, the other with the insurgent. The insurgent calls the phone with the bomb, setting it off. How do you stop it? Radio jammers.

Ms. PIATESKI: If the cell phone that is with the triggerman cannot communicate with the cell phone that's in place with the device, then obviously, the device can't be detonated.

AUSTER: Does the other side then have something they go to next?

Ms. PIATESKI: There are other forms of simple they can use.

AUSTER: Erin Piateski shows me an even simpler kind of IED: a pressure plate made of two thin hacksaw blades.

Ms. PIATESKI: It's two pieces of metal. You have these wooden spacers at the end keeping the metal apart, and it's connected to a cord. And that's it.

AUSTER: And you can't jam it. The next vehicle that drives over it sets off the bomb. How do you stop that? Piateski says you have to hope you spot it.

Ms. PIATESKI: And then you can bring a bomb squad team to come in and defuse it.

AUSTER: A bomb squad team, which brings us back to Aberdeen, where we meet with a bomb disposal team who thought it would be fun to outfit me in a bomb suit.

Unidentified Man #2: Now we're going to try to do this quick, because you start to get tired. That's when this stuff gets on.

AUSTER: Really?

Unidentified Man #2: Yeah, 85 pounds.

AUSTER: Right now, it's like a big thing of overalls with a nice green color, black boots. It's all one piece. It's taking two people to strap me into it.

AUSTER: Snap my wrists, he says, better to just fall. Not too dangerous, really, compared to what these soldiers do. When someone in Iraq or Afghanistan spots what they think is a bomb, these soldiers have to get rid of it, wearing heavy, hot suites like I tried on. Sometimes they just have to detonate the bomb. Usually, though, its' better to try to disarm the IED, so they can learn about it. Bombs have their one signature, and if they can read it, these soldiers play their part in the cat-and-mouse game.

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